1. Field of the Invention
The present invention relates to an optical semiconductor device and a method for manufacturing the optical semiconductor device.
2. Related Background Art
A photodiode of a PIN structure is conventionally employed as a photo detector which converts an optical signal used in optical communication or a DVD and the like into an electrical signal.
The PIN-type photodiode has a structure in which a so-called i (intrinsic) layer consisting of a semiconductor having a relatively low impurity concentration is put between p and n semiconductors having relatively high impurity concentrations.
A bipolar transistor, a capacitor, a resistance, a MOSFET and the like are used as signal-processing circuit elements which process an electrical signal from the photo-detector.
An optical semiconductor device is conventionally formed by hybridizing a PIN photodiode and signal-processing circuit elements formed on different semiconductor substrates or semiconductor chips, respectively (such optical semiconductor device will be referred to as “hybrid-optical semiconductor device” hereinafter).
Further, there is known an optical semiconductor device which has a PIN photodiode and a signal-processing circuit formed on the same semiconductor substrate or semiconductor chip (such optical semiconductor device will be referred to as “single-substrate-type optical semiconductor device” hereinafter).
The single-substrate-type optical semiconductor device has fewer parts than those of the hybrid-optical semiconductor device in an assembly process and requires fewer steps in the assembly process. Therefore, the single-substrate-type optical semiconductor device can reduce manufacturing costs more than the hybrid-optical semiconductor device. Further, the single-substrate-type optical semiconductor device does not require a bonding wire that connects from a semiconductor chip on which a PIN photodiode is formed to a semiconductor chip on which a signal-processing circuit is formed. Therefore, the single-substrate-type optical semiconductor device can resist external electromagnetic noise better than the hybrid-optical semiconductor device. As a consequence, the single-substrate-type optical semiconductor device is more advantageous than the hybrid-optical semiconductor device.
FIG. 8 is a schematic enlarged cross-sectional view of a conventional single-substrate-type optical semiconductor device. As shown therein, a p−-type epitaxial layer 12 is formed on a p-type semiconductor substrate 10. An n-type epitaxial layer 16 is formed on the epitaxial layer 12. An insulating layer 18, an insulating layer 20, an electrode layer 22, a passivation film 24 and a passivation film 26 are sequentially provided on the epitaxial layer 16 in this order.
On the epitaxial layers 12 and 16, various diffused layers 14, 40, 42 and 44 are provided to form a photodiode section 50 and a signal-processing circuit section 60. In addition, electrodes 28 and 29 connected to the diffused layers through the insulating layer 18 are formed on the epitaxial layers 16.
The electrode layer 22 is a metal layer electrically connected to one of the electrodes formed on the epitaxial layer 16 and also functions as a light-shielding film which shields the signal-processing circuit section from light. Therefore, in the optical semiconductor device 200, the electrode layer 22 is not formed in the photodiode section 50 and light is allowed to be incident only on this photodiode section 50.
However, the insulating layers 18 and 20 and the passivation films 24 and 26 used to manufacture the signal-processing circuit section 60, the electrode 28 and the like are formed on the surface of the epitaxial layer 16 in the photodiode section 50. Because of the presence of the insulating layers 18 and 20 and the passivation films 24 and 26, most of the incident light incident on the photodiode section 50 is reflected. As a result, the quantity of light incident on portions below the surface of epitaxial layer 16 is decreased. Due to this, the photo sensitivity of the optical semiconductor device 200 disadvantageously deteriorates.
Furthermore, the film formed on the surface of the epitaxial layer 16 in the photodiode section 50 is a multilayer film which consists of the insulating films 18 and 20 and the passivation films 24 and 26 different from one another in property and thickness. Since the respective films of this multilayer film are formed in different manufacturing steps from one another, the material, property and film thickness vary among these films. As a result, the reflectance of the incident light incident on the photodiode section 50 is not kept constant. Due to this, there occurs the problem that the photo sensitivity of the optical semiconductor device 200 has a disadvantageously large variation.
As stated above, the reflectance for reflecting the incident light incident on the photodiode section 50 is largely influenced by the materials, properties and thicknesses of the films covering the surface of the epitaxial layer 16. However, it is difficult to form the films having different materials, properties and thicknesses on the epitaxial layer 16 so as to minimize reflectance in view of the refractive index of the epitaxial layer (e.g., the refractive index of silicon≈3.44) and the wavelength of the incident light.
In addition, Japanese Patent Application Publication No. 4-271173 discloses an optical semiconductor device having a dielectric thin film and an antireflection film which have common properties and thickness, and which are manufactured in a common manufacturing step. The dielectric thin film is used between the electrodes of the capacitor of a peripheral circuit element. The antireflection film is used in a photo detector.
In the optical semiconductor device disclosed in Publication No.HEI4-271173 (1992), however, the thickness of the antireflection film is a factor that determines the capacitance of the capacitor. Therefore, the thickness of the antireflection film is limited by the capacitance of the capacitor. If the thickness of the antireflection film is set at an optimum thickness in accordance with the wavelength of incident light, the areas of the electrodes of the capacitor have to be changed so as to obtain a desired capacitance.
Furthermore, in the optical semiconductor device disclosed in Publication No. 4-271173, the antireflection film of the photo detector is formed when the dielectric thin film used between the electrodes of the capacitor is formed. Due to this, such films as passivation films are formed on the antireflection film of the photo detector. As a result, there occurs the problem that in order to control the reflectance in the photo detector, it is disadvantageously necessary to control not only the thickness of the antireflection film but also that of the passivation films on the antireflection film.
Therefore, it is desired to provide an optical semiconductor device which has a relatively high photo sensitivity and which can reduce the variation of photo sensitivity even if a photo detector and a circuit element are formed on the same semiconductor substrate, and to provide a method for manufacturing the optical semiconductor device.
It is also desired to provide an optical semiconductor device which can control a photo sensitivity relatively easily without influencing a circuit element even if a photo detector and a circuit element are formed on the same semiconductor substrate, and to provide a method for manufacturing the optical semiconductor device.
It is further desired to provide a method for manufacturing an optical semiconductor device which enables a photo detector and a circuit element having relatively high photo sensitivity and small variation in photo sensitivity to be manufactured on the same semiconductor substrate, and to provide the optical semiconductor device.